| The drilling riser is the connection between a platform on the water surface and the well head on the sea floor, which is used to protect drilling pipe and transportat the drilling mud. The deeper water drilling riser can be treated as an extremely long and flexible tensioned prismatic tube that is the weaknesses of entire drilling system. It is exposed to the harsher marine environment, meanwhile subjected to a time-varying distributed load due to the ocean current, the main failure of riser is corrosion and corrosion fatigue and is usually hailed as"throat"in the development of deep-water drilling. At present, the widely used organic coating protection is far from meeting the requirements of riser design life, in view of the maintenance inconvenient and high maintenance coasts of marine engineering structures, there is an urgent need to develop a more long-lasting corrosion protection coating.Aluminum, Al/Zn pseudo alloy, Zn-15wt%Al alloy and Zn+Al composite coating were deposited on steel substrate using arc spraying, and some specimens of each coating were sealed with epoxy primer and polyurethane topcoat. Laboratory coupon test, out sea in-situ plates test together with electrochemical test were carried out to evaluate the coating corrosion resistance of different coatings. Optical microscopy, three-dimensional stereo microscope and Scanning Electron Microscope (SEM) were used to observe the corrosion morphology and analyze the change in microstructure of various coatings. The results showed that the corrosion resistance of Al/Zn pseudo alloy coating is the best among the above-mentioned four coatings. Pitting corrosion appeared in aluminum coating, and Zn-15wt%Al alloy coating was proned to peeling off, while Zn+Al composite coating bulged seriously. Sealing with organic coating could greatly improve the corrosion resistance of bare metallic coating, and Al/Zn pseudo alloy coating had the best compatibility with its sealing paint. Marine biological attachment tended to decrease with the increase in zinc content of coatings.The corrosion failure process of aluminum coating was investigated by means of electrochemical impedance spectroscopy (EIS) measurements. Observing the change in impedance diagram during corrosion test, it could be divied into four stages: (1) pitting initiation; (2) hydrogen evolution resulted from acidification in the pits; (3) coating acts as the sacrificial anode after the corrosion medium penetrates into interface between the coating and steel substrate; (4) rapid development of pitting groups. For Al/Zn pseudo alloy and Zn-15wt%Al alloy coating, contraction in the low-frequency real part of Nyquist plots was always found during EIS testing process, which was mainly due to selective corrosion of the coating.A cantilever bending corrosion fatigue testing machine was designed and constructed in this paper in order to simulate the service environment of deep sea drilling riser. The testing machine can be achieved in adjustude amplitude and adjustable frequency. The corrosion fatigue tests of coated steel and bare steel were carried out in 3.5%NaCl solution, evaluating the effects of coating on the corrosion fatigue life of steel. The results show that Al/Zn pseudo alloy coating could significantly improve the corrosion fatigue life of steel substrate.
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